Scientists have figured out how the AIDS virus deceives antibodies when it invades healthy cells.

Duke University Medical Centre researchers in the US said some of the earliest and most abundant antibodies available to fight HIV can't actually "see" the virus until after it has already invaded a healthy cell.

Previous research had shown that two of the most robust antibodies against HIV -- called 2F5 and 4E10 -- target a specific part of the outer coating of the virus called the MPER region of gp41, the journal Nature Structural & Molecular Biology reports.

The antibodies are able to latch on to this part of the virus, referred to as its "Achilles heel".

"What our studies revealed, however, is that the virus actually creates two versions of this 'Achilles heel'," says Duke Human Vaccine Institute (DHVI) director Barton Haynes who was the senior study author, according to a Duke release.

"One version is for these rarer, broadly-neutralising antibodies, and the other is for the more abundant, first-responding antibodies that won't be able to do much good because the Achilles heel isn't detectable to them until the virus has already gained entry."

Haynes says the findings are important because they distinguish what parts of the virus an antibody needs to recognise from those parts that are decoys.

Nathan Nicely led the study and as member of the DHVI designed and conducted most of the crystallography studies. The study was funded by a Collaboration for AIDS Vaccine Discovery Grant from the Bill & Melinda Gates Foundation.